New evidence suggests that repetitive hits to the head, even if they don't cause concussions, can cause brain damage and contribute to the degenerative brain disease CTE. In two of the other cases, abnormal levels of tau protein - which can kill brain cells and is a marker of CTE - accumulated inside their brains. The disease also affects military veterans who suffered brain injuries since September 11, 2001. In the past, concussions were seen to be the major cause of CTE, but a new study suggests that hits to the head triggers CTE, not concussions. There have been studies earlier that show that CTE has occurred without the signs of a concussion. As it happens, CTE is characterized by the accumulation of tau proteins around the blood vessels.

Dr. Lee Goldstein is a researcher at Boston University where he specializes in degenerative orders, specifically Alzheimer's disease - which shares numerous same symptoms as CTE.

Researchers hypothesized that early CTE may result from damaged blood vessels in the brain that become leaky, resulting in blood proteins spilling into brain tissue and triggering brain inflammation, according to the study. The brains of those who did not experience head injuries did not have the same signs, the researchers state.

"The same brain pathology that we observed in teenagers after head injury was also present in head-injured mice", Goldstein told BU Today. They administered a few mild blows to the heads of the lab mice: the force was equivalent to the impact of a right hook to a human, or a helmet-to-helmet football collision, but the blows did not always cause concussions in the mice, says Goldstein.

Nowinski, a former professional wrestler who is now a neuroscientist at Boston University, noted that the U.S. Soccer Federation forbids kids younger than 11 from heading the ball, and that USA Hockey outlawed checking in the sport for kids younger than 13. Until now, the focus was on any concussions they received.

Individuals suffering from CTE will suffer gradually deterioration of their brains and will end up losing brain mass over the years or decades.

"Kids are getting hit and hurt, but they aren't getting helped", Goldstein said.

"In order to reduce CTE risk" in athletes and military veterans, "there must be a reduction in the number of head impacts", said McKee, who is the director of the CTE Center.

"At some point, those of us who have had success in this game must speak up to protect both football players and the future of the game, and supporting "Flag Football Under 14" is our best way to do that", he said.